Electrical discharge machining (EDM) is a well-known process for machining an electrically conductive workpiece or part sometimes performed by a numerically controlled device. EDM is described in Chapter 14, Volume 1 of Tool and Manufacturing Engineers Handbook (1983), by Thomas J. Drozda and Charles Wick, published by the Society of Manufacturing Engineers (SME) and herein incorporated by reference. Other features of EDM are described in "Fast EDM Drilling," by Peter Roy Durgan, a technical paper published in 1989 by the SME, "Multi-Small Hole Drilling by EDM," by D. F. Troller, a technical paper published in 1983 by the American Institute of Aeronautics and Astronautics, "Small Hole Drilling in EDM," by M. L. Jeswani, published in the International Journal of Machine Tool Design, Volume 19, pp. 165-169 in 1979; "Study on Micro-Hole Drilling by EDM," by T. Masuzawa, M. Fujino, K. Kobayashi, and T. Suzuki, published in the Bulletin of the Japanese Society of Precision Engineering, Volume 20, No. 2, in June 1986; "High-Volume Hole Making with EDM," by R. L. Hatschek, published in American Machinist in October, 1983; "The Performance of Single-Electrode Fine-Hole Drilling by Electro-Discharge Machining," by J. R. Crockall and M. I. Kamal, published in 1976 in the Proceedings of the 17th Annual Machine Tool Design and Research Conference; and "The Development of an Electrodischarge Machine for Micro-Hole Boring," by T. Sato, T. Mizutani, K. Yonemochi, and K. Kawata, published in Precision Engineering, Volume 8, No. 3, in July 1986, all of which are herein incorporated by reference.
Specifically, as described in the Hatschek article, such machining includes forming at least one and frequently a large number of cooling holes or apertures in combustor liners for aircraft jet engines. During EDM, holes are formed by vaporizing a small portion of electrically conductive material, such as metal, with an electrical discharge. An EDM apparatus typically includes one or more electrodes for conducting electrical discharges between the apparatus and the part.
The EDM process described above has several problems. First, as a result of the electrical discharges, the electrodes of the EDM apparatus erode to a bullet shaped tip that requires drilling a predetermined distance beyond the thickness of the part to obtain a through hole of proper size and shape. Second, beyond the combustor liner wall is another wall, called a plenum wall, that due to various process anomalies or errors may be machined or drilled by the electrodes unintentionally. Drilling of the plenum or other back wall is referred to as "scarfing." Such damage to the part or workpiece may entail costly repair procedures or result in the scrapping of valuable hardware. A third problem relates to regulation of the feedrate of the electrodes. If the electrodes are too far from the part, no electrical discharge will take place Likewise, if the electrodes are too close or even touch the part no discharge occurs due to an electrical short. Thus, to perform EDM the electrodes must stand off a predetermined distance from the metal part. In this context, the shortest distance between the electrode and the part when the electrode is positioned in a particular location adjacent the part constitutes the standoff distance.
U.S. Pat. No. 4,495,394, "Electronic Depth Controller for EDM Apparatus," by McGregor et al., issued Jan. 22, 1985, assigned to Raycon Corporation and herein incorporated by reference, discloses an electronic depth controller for an EDM apparatus based upon the output of a position transducer. Although this type of control may reduce the amount of scarfing, it does not detect the onset of scarfing. A need thus exists for a real-time closed loop feedback control system to detect the onset of scarfing.